1. Field of the Invention
This invention relates to an oscillator circuit using a piezoelectric resonator and more particularly to an oscillator circuit using a crystal and metal oxide semiconductor field effect transistors (hereinafter referred to as MOS FET).
2. Description of the Prior Art
As an integrated circuit used in an electronic timepiece or the like for which low power consumption is required, there has conventionally been used a complementary field effect transistor (hereinafter referred to as a C-MOS FET) circuit which comprises principally a series connection of a p-channel MOS FET and an n-channel MOS FET. A piezoelectric resonator such as a crystal resonator has also gained wide application to oscillator circuits because it enables the oscillation frequency to be accurately determined.
A conventional oscillator using these C-MOS FET circuits and a piezoelectric resonator generally has a closed circuit formed by a single-stage C-MOS inverter and a crystal resonator as illustrated in FIG. 1. In this case, the output waveform does not assume a perfectly rectangular shape because the gain of the inverter is not large enough. Consequently, the current flowing through the inverter becomes large and power consumption becomes high. It is therefore impossible to operate the circuit at a low rate of power consumption.
To minimize the power consumption, it is also known to use three-stage C-MOS inverters connected in a cascade connection instead of the single-stage C-MOS inverter thereby to separate the amplification stage from the output stage. In this case, since a sufficient gain can be obtained, the output waveform assumes a perfectly rectangular shape and the power consumption becomes low during the stable oscillation. In more detail, the current flowing through the final inverter stage as an output stage is minimized by the rectangular shape output. However, due to the high gain, there tends to be formed a CR oscillation circuit in which the crystal resonator operates as a capacitor C.sub.5 in a feed-back loop. This circuit causes CR oscillation at a frequency deviated considerably from a predetermined resonance frequency when the operating voltage rises abruptly, especially during the transient period such as at start of oscillation. This abnormal oscillation often occurs at a frequency considerably higher than the oscillation frequency of the crystal resonator. For this reason, it has been impossible to obtain stable oscillation at the start of oscillation.